Conventionally, aluminum hydroxide has been employed as filler for fulfilling various functions in rubber/plastics. For example, aluminum hydroxide is added, as a flame retardant, to thermoplastic resins, rubber or epoxy resins, and is added, as color-controlling filler, to thermosetting resins, such as unsaturated polyester resins and acrylic resins.
When aluminum hydroxide is employed as a flame retardant, flame retardation performance is enhanced as the amount of aluminum hydroxide is increased. However, addition of a large amount of aluminum hydroxide elevates the torque required for kneading and the molding temperature, thereby causing problematic foaming due to dehydration of a portion of aluminum hydroxide. When aluminum hydroxide is added to thermosetting resins, an increase in the amount of the added aluminum hydroxide reduces material costs, but material strength problematically decreases.
In order to prevent decrease in material strength, particle size is desirably reduced to be as small as possible. Although aluminum hydroxide particles having a small particle size can be yielded through precipitation, addition of a large amount of such small particles serving as filler is difficult, since aluminum hydroxide forms agglomerated secondary particles, which are formed through agglomeration of a large number of primary particles, and exhibits considerably high absorption of oil. Accordingly, aluminum hydroxide particles having a particle size of approximately 50 to 150 μm are pulverized by means of a ball mill or other pulverizer to thereby form approximately primary particles, which are generally employed as filler.
However, pulverizing the particles to a predetermined particle size through a pulverization technique requires a large amount of energy. In addition, aluminum hydroxide primary particles yielded through pulverization are caused to break, thereby causing roughening of particle surfaces, chipping of the particles, etc. As a result, the BET specific surface area of the resultant powder increases. Thus, such powder has poor compatibility with resin and increases the viscosity of the resin containing the powder, failing to attain high-density incorporation of the powder. When the powder is added to a thermosetting resin, the curing time of the resin is prolonged.
On the basis of these tendencies, aluminum hydroxide ideal for serving as filler is thought to have a small surface roughness, i.e., a small BET specific surface area, and to form separate, individual particles.
JP-B HEI 5-4336 discloses a method for disintegrating agglomerated secondary particles by application of strong centrifugal force by means of a continuous screw decanter without breaking the corresponding primary particles, thereby preventing roughening of surfaces of the primary particles. However, the above method is limited to applications of a specific raw material. That is, the method cannot be applied to a wide range of materials.
JP-B SHO 62-9256 discloses a method for producing single-crystalline or roundish aluminum hydroxide particles by bringing a temperature-elevated Bayer extract into contact with solid aluminum hydroxide. However, the above method has drawbacks in that it requires a long period of contact time and that dissolution of aluminum hydroxide during contact is promoted, thereby deteriorating production efficiency.
JP-A HEI 9-208740 discloses a method for reducing the BET specific surface area of aluminum hydroxide particles by pulverizing in advance agglomerated aluminum hydroxide secondary particles by means of a dry-impact pulverizer, adding the pulverized product into a sodium aluminate solution having a predetermined alkaline concentration to thereby form a slurry, and elevating the temperature of the slurry to thereby dissolve the particle surfaces. However, the method also has a drawback in that aluminum hydroxide has to be filtered and dried in order to effect dry pulverization performed in advance, thereby prolonging production steps and elevating production costs.
Accordingly, an object of the present invention is to provide aluminum hydroxide ideal for serving as filler, the aluminum hydroxide having a small BET specific surface area over a wide particle size range and forming separate, individual particles.
Another object of the invention is to provide a method for effectively producing the aluminum hydroxide.
In view of the foregoing, the present inventors have carried out extensive studies in order to attain the above objects, and have found that ideal aluminum hydroxide having a small specific surface area suitable for serving as filler and forming separate, individual particles can be produced through a combination of a first step of elevating the temperature of a slurry under predetermined conditions, which slurry is yielded by suspending aluminum hydroxide in a specific sodium aluminate solution, and a second step of applying a centrifugal force to the slurry to thereby form a sediment having an elevated solid content and disintegrate agglomerated secondary particles. The present invention has been accomplished on the basis of this finding.